This invention relates to spark timing systems for spark ignition, internal combustion engines and particularly to such spark timing systems including a crankshaft driven source of alternating current ignition signals and means to switch current on and off in a spark coil in response to such signals to generate spark pulses for application to the spark plugs at normal predetermined times with respect to crankshaft rotation.
It may be desirable to selectively retard the spark pulses from the normal predetermined spark timing under certain conditions. As example is found in the advancement of normal predetermined spark timing for greater power and fuel economy or addition of an engine turbocharger and the corresponding addition of a system effective to automatically retard the spark timing upon the sensing of engine knock or certain engine operating conditions conducive to engine knock. For example, a system capable of measuring engine knock intensity, developing an output signal in response thereto, and applying the signal to retard engine spark timing is shown and described in the U.S. Pat. No. 4,002,155, to Harned et al issued Jan. 11, 1977.
However, retard in any spark pulse follows the normal coil-on signal from the alternating current signal generator. Therefore, when engine speed is high and the retard is lengthy, the next coil-on signal from the alternating current signal generator may follow the retarded spark pulse very closely and, in extreme conditions, may even precede the retarded spark pulse. As a result, the spark burn time or conduction time of the spark plug, which requires the conduction in the spark coil primary to be cut off, may be too short to ensure complete combustion in the cylinder.